Recently much attention has been focused on fuel cells that feature not only high energy conversion efficiency but also no hazardous substance produced by the electricity-generating reaction. Known as one of such fuel cells is the polymer electrolyte fuel cell which operates at a low temperature of 100° C. or below.
A polymer electrolyte fuel cell, which has a basic structure of a solid polymer electrolyte membrane disposed between a fuel electrode and an air electrode, generates power through an electrochemical reaction as described below by supplying a fuel gas containing hydrogen to the fuel electrode and an oxidant gas containing oxygen to the air electrode.Fuel electrode: H2→2H++2e−  (1)Air electrode: (½)O2+2H++2e−→H2O   (2)
The anode and the cathode have each a stacked structure of a catalyst layer and a gas diffusion layer. And a fuel cell is composed of catalyst layers of the respective electrodes disposed counter to each other in such a manner as to hold a solid polymer membrane therebetween. The catalyst layer is a layer of a catalyst or carbon particles carrying a catalyst bound together by an ion-exchange resin. The gas diffusion layer serves as a passage for the oxidant gas or the fuel gas.
At the anode, the hydrogen contained in the supplied fuel is decomposed into hydrogen ions and electrons as expressed in the above formula (1). Of them, the hydrogen ions travel inside the solid polymer electrolyte membrane toward the air electrode, whereas the electrons travel through an external circuit to the air electrode. At the cathode, on the other hand, the oxygen contained in the oxidant gas supplied thereto reacts with the hydrogen ions and electrons having come from the fuel electrode to produce water as expressed in the above formula (2). In this manner, the electrons travel from the fuel electrode toward the air electrode in the external circuit, so that the electric power is extracted therefrom (See Patent Document 1).
In order to simplify the polymer electrolyte fuel cell system for home use and reduce the cost thereof, a membrane electrode assembly (MEA), which is the power section of the fuel cell system, requires the robustness against the temperature fluctuations of the humidifying temperature and the cell temperature as well as the durability. In currently available MEA, the voltage also fluctuates when the humidifying temperature and/or the cell temperature fluctuate. Also, it is known that the rate of voltage drop is high relative to a conventional low-humidified continuous operation. As a way of addressing these problems, in Patent Document 2, a humidity control agent, such as mesoporous silica, is disposed in an interface between an electrolyte membrane and a catalyst layer and an interface between a catalyst layer and a gas diffusion layer, and is also disposed in layers outside the gas diffusion layer or the like. This humidity control agent keeps the interior of the fuel cell at a constant humidity and humidifies the solid polymer electrolyte membrane appropriately, so that a fuel cell, which does not require any auxiliary device, has been further conceived.
[Patent Document 1] Japanese Patent Publication No. 2002-203569.
[Patent Document 2] Japanese Patent Publication No. 2002-270199.